After growth, the isolates were centrifuged at 4000 rpm for
15 min. The pellet was resuspended in peptone water and inoculated
(1% (w/v)) in 10 mL whole UHT milk and incubated at 30 C for
24 h. Then, 1 mL of culture was added to 0.5 mL of a-naphthol
solution (1% (w/v)) and KOH (16% (w/v)) followed by incubation at
37 C for 10 min. The production of diacetyl was indicated by a
formation of a red ring in the tubes (King, 1948). The results were
classified as weak, medium or strong according to the intensity of
the colour of the ring formed. The tests were conducted in
duplicate.